Metering gear pumps operate by squeezing out accurate volumes of liquid between meshing gears. Typically, the gears are mounted within stacked plates that are appropriately ported to receive liquid between the gears and discharge the liquid in one or more streams.
In highly accurate gear pumps useful for metering resins, for example, tolerances are critically maintained between pump bearings and shafts as well as between support plates (often referred to as "kidney plates") and gears. Theoretically, these part relationships maintain so-called "zero clearance." Due to the close tolerances maintained on the various internal components of the gear pump, frictional heat build-up can become a significant problem. At times, this can cause relatively moving parts to adhere to one another when dispensing materials such as certain epoxy resins. This problem is believed to be most prevalent in areas of the pump where there is little or no clearance between relatively moving parts and little or no flow of liquid, such as resin, to act as a lubricant or coolant. In this latter regard, undesirable temperature increases between the gears and the support plates can be prevented by resin flowing through such areas to essentially act as a lubricant or coolant. However, frictional temperature increases between a bearing surface and associated shaft, for example, may be much higher due to the combination of close dimensional tolerances and a low flow or amount of resin between these components. Particularly when both the bearing and associated shaft are formed of metal, this heat build-up can cause the associated bearings and shafts to adhere to one another and decrease pump performance. The resulting pump downtime and maintenance or replacement of the pump considerably increases costs to the user.
In one type of metering gear pump, the above-mentioned problem has been experienced between an idler gear and its associated shaft when these components move relative to each other, and also between the various shafts and their associated bearings or bushings, which also move relative to each other. While non-metallic parts, such as ceramic bushings, have been used to reduce the problem, the use of such parts throughout the pump may not be practical. This experience prompted the need to evaluate the effect of resins on the internal moving parts of a pump and, specifically, evaluate situations in which the internal parts move relative to each other with very close tolerances. Solving the problems related to heat build-up and particularly part adherence in such pumps will result in pumps requiring lower maintenance and having longer useful lives.